Conventionally, various starters for cranking an internal combustion engine have been known. For instance, as disclosed in Japanese patent laid open publication No. 61-53568, and U.S. Pat. Nos. 4,604,907, 4,561,316, 4,573,364, 4,520,285, 4,510,406, and 4,528,470, a pinion is selectively meshed with a ring gear of the engine to crank the same according to the on-off action of an electromagnetic switch with a DC motor which drives the pinion via a planetary gear reduction unit.
In such a starter, the casing accommodating the pinion is fitted into a mounting bore of a crank case of an engine and is secured therein by fastening threaded bolts passed through a flange portion of the casing in order to ensure the high precision of the meshing between the ring gear and the pinion. Further, a mounting end surface for securing purpose is provided in the flange portion around each of the holes for passing a threaded bolt as a planar surface perpendicular to the axial line of the output shaft, and this mounting end surface is brought into contact with an associated mounting surface of the crank case so that the misalignment of the output shaft at the time of mounting may be avoided.
When the pinion is meshed with the ring gear to crank the engine, the load acting on the pinion is transmitted to the mounting end surface through a certain lever action with the fitted portion or a circumferential mounting surface of the starter casing serving as a fulcrum. According to the above described conventional structure, since the fitting boss portion serving as the mounting circumferential surface extends continuously from the mounting flange portion provided with the mounting end surfaces, a substantial bending moment acts upon the flange and a relatively large load acts upon the mounting end surfaces due to the reaction force acting on the pinion as it cranks the engine. Therefore, in order to ensure a sufficient rigidity of the flange portion, it was necessary to increase the thickness and size of the flange to an undesirable extent.
In assembling such a starter, the casing is supported typically by placing a pair of mounting end surfaces for securing purpose, provided in flanges disposed in diagonally opposing positions on the casing accommodating a pinion, on an assembly jig. Since the mounting end surfaces are provided in diagonally opposing, 180 degree opposed positions around the drive shaft, the axial force applied to the drive shaft when fitting it into the bearing of the casing may be supported by the mounting end surfaces in a stable fashion.
In terms of the freedom in designing the mounting structure between the starter and the engine, it is preferable to arrange holes for passing fastening bolts in mutually asymmetric positions with respect to the drive shaft. However, if the mounting end surfaces defined around such mounting holes are arranged in mutually asymmetric positions with respect to the drive shaft, the force applied to the drive shaft to fit the drive shaft into a bearing provided in the casing while supporting its mounting surfaces with an assembly jig produces a moment, and it impairs the efficiency of the assembly work.